This invention relates to a flowmeter and, more particularly, to a flowmeter of the magnetic induction type which measures the rate of flow through a pipe by applying a magnetic field to the flowing liquid in the pipe and then detecting the resulting induced voltage generated between two probes in contact with the flowing liquid.
When a magnetic field is applied to an electrically conducting flowing liquid, the liquid will act as an electrically conducting wire moving through a magnetic field and generate a voltage across the body of the liquid as it moves through the applied magnetic field. The voltage will be generated perpendicular to the applied magnetic field. Thus, if the magnetic field is applied to a pipe section through which an electrically conducting liquid is flowing and two probes are provided to be in electrical contact with the liquid at points on opposite sides of the pipe positioned on a line perpendicular to the magnetic field, a voltage will be generated on the probes, which voltage will vary with the rate of flow. It has been proposed in the prior art to make use of this phenonomena to measure the rate of flow through a pipeline by using a DC magnetic field. However, the voltage produced by the probe is subject to large drifts in value and the proposed flowmeter failed to provide a reliable indication of the flow rate. To overcome the problem of voltage drift, an AC magnetic field was applied to the pipe section through which the flow rate is to be measured and the resulting AC induced voltage was detected and filtered at the frequency of the applied AC field to eliminate the effect of DC drift. This flowmeter of the prior art provided a reliable indication of the flow rate, but the electromagnet in order to produce an output siganl of sufficient magnitude was large and cumbersome and required a large amount of power to generate the alternating field. Another system of the prior art, instead of applying an alternating field to the pipe section, applied a DC field by means of an electromagnet and the output voltage from the applied DC field was stored. Subsequently, the DC field applied by the electromagnet was reversed and the output voltage was again detected. The difference in the two output voltages detected was then measured to provide an indication of the flow rate. This system also required large electromagnets and required a substantial amount of power to reverse the fields in a short enough period of time to achieve a reliable flow rate measurement.